In acoustic equipment having a speaker as a load, recently, a D-class or digital amplifier has been in practical use to amplify audio signals recorded on optical diskettes such as CDs or compact disks, and an electric power converter has been utilized which comprises a switching power generator for supplying electric power from AC power source to D-class amplifier. Such a power converter incorporated with D-class amplifier and switching power generator is superior in power conversion efficiency to produce larger power signals in small size of the converter, compared to prior art power converter incorporated with low frequency transformer and analog amplifier.
FIG. 10 shows an example of prior art power converters utilizing a switching power generator and two D-class power amplifiers connected to the subsequent stage of the switching power generator. Switching power generator of prior art converter shown in FIG. 10 comprises a DC power source 1 which comprises a rectifying bridge circuit 1b connected to a commercial AC power source 1a and an input smoothing capacitor 3 connected to bridge circuit 1b; a primary winding 9a of a transformer 9 and a main MOS-FET 4 as a main switching element connected in series to one and the other ends of input smoothing capacitor 3; a positive output commutating diode 26 having the anode terminal connected to one end of a secondary winding 9b of transformer 9; a positive output smoothing capacitor 27 connected between cathode terminal of commutating diode 26 and a grounded center tap 9e of transformer 9; a negative output commutating diode 28 having the cathode terminal connected to the other end of secondary winding 9b of transformer 9; a negative output smoothing capacitor 29 connected between anode terminal of commutating diode 28 and grounded center tap 9e of transformer 9; voltage-detecting resistors 30, 31 connected in parallel to output smoothing capacitors 27 and 29 connected in series to each other; an error amplifier 33 for producing a signal of differential voltage between a junction voltage VA of voltage-detecting resistors 30, 31 and a reference voltage VR from a normal power source 32; a main control circuit 6 for producing drive signals to main MOS-FET 4; a photo-coupler 34 which has a light emitter 34a and a light receiver 34b for transmitting differential voltage signal from error amplifier 33 to a feedback signal input terminal FB of main control circuit 6 which turns main MOS-FET 4 on and off based on differential voltage signal from error amplifier 33 to feedback terminal FB through photo-coupler 34, and an auxiliary power source 7 for supplying DC voltage VCC to a power input terminal VCC of main control circuit 6 for activation.
Each of power amplifiers connected to the output stage of the switching power generator, comprises first and second regulatory MOS-FETs 10, 11, 35, 36 as first and second regulatory switching elements connected to respectively upper end of positive smoothing capacitor 27 and lower end of negative smoothing capacitor 29 and connected in parallel to voltage-detecting resistors 30 and 31; a choke coil 13, 38 having one end connected to a junction of first and second regulatory MOS-FET 10, 11, 35, 36; a filtering capacitor 14, 39 connected between the other end of choke coil 13, 38 and source terminal of second regulatory MOS-FET 11, 36; a speaker 15, 40 as a load connected in parallel to filtering capacitor 14, 39; dividing resistors 16, 17, 41, 42 connected between a junction of choke coil 13 and filtering capacitor 14 and ground; an output error amplifier 18, 43 for producing a signal of potential difference between a voltage of an external input signal VIN1, VIN2 and a divided voltage VO1, VO2 on junction of dividing resistors 16, 17, 41, 42; a PWM modulator 19, 44 as a pulse modulating circuit for receiving outputs from output error amplifier 18, 43 to produce PWM (pulse width modulation) signals with the varied pulse duration; positive and negative waveform shapers 20, 21, 45, 46 for adding dead times to PWM signals from PWM modulator 19, 44; an inverter 22, 47 for producing inverted signals of output signals from positive adder 21, 46; a level shift circuit 23, 48 for elevating or boosting voltage level of output signals from inverter 22, 47 by the potential applied on junction of first and second regulatory MOS-FETs 10, 11, 35, 36; a negative drive circuit 24, 49 for receiving output signals from negative waveform shaper 20, 45 to provide drive signals for a gate terminal of second regulatory MOS-FET 11, 36 for the on and off operation thereof; and a positive drive circuit 25, 50 for receiving output signals from level shift circuit 23, 48 to provide drive signals for a gate terminal of first regulatory MOS-FET 10, 35 for the on and off operation thereof. Choke coil 13, 38 and filtering capacitor 14, 39 constitute a low pass filtering circuit for allowing only sine wave signals of audible frequency bandwidth to pass therethrough. In the circuitry shown in FIG. 10, applied to non-inverted input terminal + of output error amplifier 18, 43, are audio or voice input signals as external input signals VIN1, VIN2 output from for example a microphone or CD player not shown.
In operation of prior art power converter shown in FIG. 10, AC voltage E from commercial AC power source 1a is subject to full-wave rectification through rectifying bridge circuit 16, and then smoothed through smoothing capacitor 3 into a DC voltage. When main control circuit 6 delivers drive pulse signals to gate terminal of main MOS-FET 4 to turn main MOS-FET 4 on and off, DC voltage is intermittently applied from smoothing capacitor 3 to primary winding 9a of transformer 9 to produce DC voltage of rectangular waveform across primary winding 9a. Accordingly, induced at both ends of secondary winding 9b of transformer 9 is AC voltage of rectangular waveform which is then converted into positive and negative DC voltages by two rectifying smoothers, one having combined positive commutating diode 26 and positive smoothing capacitor 27, and the other having combined negative commutating diode 28 and negative smoothing capacitor 29. Specifically, while DC voltage of positive polarity is produced across or between opposite ends of positive smoothing capacitor 27, DC voltage of negative polarity is produced across or between opposite ends of negative smoothing capacitor 29. Detecting resistors 30, 31 pick out voltage developed between upper end of positive smoothing capacitor 27 and bottom end of negative smoothing capacitor 29, and error amplifier 33 produces error voltage signals between detected voltage VA on junction of detecting resistors 30, 31 and reference voltage VR of normal power source 32. Error voltage signals activate light emitter 34a of photo-coupler 34 to transmit error voltage signals to feedback input terminal FB of main control circuit 6 through light receiver 34b so that main control circuit 6 controls duty ratio of drive pulses to gate terminal of main MOS-FET 4 based on error voltage signals to feedback input terminal FB. Thus, stabilized positive and negative DC voltages can be applied to opposite ends of positive and negative smoothing capacitors 27 and 29 to apply these DC voltages on first and second regulatory MOS-FETs 10, 11, 35, 36 of power amplifiers.
In each power amplifier, AC voltage can be applied to each speaker 15, 40 through choke coil 13, 38 and filtering capacitor 14, 39 by alternately turning first and second regulatory MOS-FETs 10, 11, 35, 36 on and off. AC voltage applied on speaker 15, 40 is split by dividing resistors 16, 17, 41, 42 to apply to inverted input terminal − voltage VO1, VO2 appearing on junction of dividing resistors 16, 17, 41, 42. Output error amplifier 18, 43 generates potential difference signals between voltage VIN1, VIN2 of external input signal applied to non-inverted input terminal + and voltage VO1, VO2 appearing on junction of dividing resistors 16, 17, 41, 42, and then, PWM modulator 19, 44 issues PWM signals the pulse length of which is varied in proportion to potential difference signals from output error amplifier 18, 43 to generate PWM signals of pulse width proportional to amplitude of external input signal VIN1, VIN2 from PWM modulator 19, 44. PWM signals from PWM modulator 19, 44 are supplied to positive and negative waveform shapers 20, 21, 45, 46 which add to PWM signals dead times during which first and second regulatory MOS-FETs 10, 11, 35, 36 are concurrently turned off. PWM signals to which negative waveform shaper 20, 45 adds dead times, are furnished to gate terminal of second regulatory MOS-FET 11, 36 through negative drive circuit 24, 49 to turn second regulatory MOS-FET 11, 36 on and off. On the other hand, PWM signals to which positive waveform shaper 21, 46 adds dead times are furnished to gate terminal of first regulatory MOS-FET 10, 35 through inverter 22, 47, level shift circuit 23, 48 and positive drive circuit 25, 50 to turn first regulatory MOS-FET 10, 35 on and off. Thus, each pulse width of PWM signals is varied based on external input signals VIN1, VIN2 from for example a microphone or CD player to alternately turn first and second regulatory MOS-FETs 10, 11, 35, 36 on and off by PWM signals with the changing pulse length so that AC pulse signals VP1, VP2 with the modulated pulse width are produced from junction between first and second regulatory MOS-FETs 10, 11, 35, 36. AC pulse signals VP1, VP2 are supplied from junction of first and second MOS-FETs 10, 11, 35, 36 to speaker 15, 40 through low pass filter of choke coil 13, 38 and filtering capacitor 14, 39 for removing high harmonic wave over audible frequency from AC pulse signals VP1, VP2 commensurate or proportional to external input signals VIN1, VIN2.
For example, Japanese Patent Disclosure No. 8-23241 represents a power amplifier which comprises an inverter for converting DC voltage into pulsatile AC voltage supplied to switches through a half-wave rectifier; a binary state modulator for converting audio signals such as voice into pulse signals to turn the switches on and off by means of these pulse signals for pulse number modulation of pulsatile AC voltage; a filter for restricting a part of bandwidth in output signals from the switches; and a demodulator for demodulating the output signals into power-amplified audio signals applied to a load through output terminals. As this power amplifier does not need conversion of output voltage from inverter into DC voltage ever once, it can dispense with smoothing capacitor advantageously with increased power conversion efficiency and with less noise because of conversion of output voltage from inverter into a sine wave.
Prior art power converter shown in FIG. 10 firstly converts AC voltage E from commercial AC power source 1a into positive and negative DC voltages through transformer 9 by turning main MOS-FET 4 of the switching power generator on and off, secondly again converts positive and negative DC voltages into pulsatile AC signal VP1, VP2 of several hundred kilohertz by turning each regulatory MOS-FET 10, 11, 35, 36 of power amplifier on and off synchronously with external input signals VIN1, VIN2 supplied from microphone or CD player, and then thirdly supplies, to speaker 15, 40 through low pass filtering circuit, AC signals of waveform analogous or similar to that of external input signals VIN1, VIN2 to regenerate amplified voice signals. In this way, the converter requires the double stages of high frequency switching operation inconveniently with associated increased switching noise and power loss by heat generated from positive and negative commutating diodes 26, 28 connected at the output stage of the switching power generator, thereby causing power conversion efficiency to drop. Also, the converter disadvantageously has the complicated circuit configuration because secondary winding 9b of transformer 9 is connected to a pair of rectifying smoothers which comprise two combinations of positive output rectifying diode 26 and smoothing capacitor 27, and negative output rectifying diode 28 and smoothing capacitor 29 to produce positive and negative DC voltages.
The power amplifier shown in the above Japanese Disclosure turns the switches on and off by means of pulse signals for pulse number modulation of pulsatile AC voltage beneficially without smoothing capacitor, however, it requires a half-wave rectifier which undesirably causes decrease in power conversion efficiency like the converter shown in FIG. 10. Moreover, the power amplifier needs a filtering circuit of a choke coil and a capacitor between output side of the switches and a load such as a speaker for demodulation into audio signals so that such a filtering circuit hinders simplification of circuit configuration.
An object of the present invention is to provide a power converter capable of improving power conversion efficiency with a simplified circuit configuration.